Microporous membranes can be used as battery separators in, e.g., primary and secondary lithium batteries, lithium polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc secondary batteries and the like. When microporous polyolefin membranes are used for battery separators, particularly lithium ion battery separators, the membranes' characteristics significantly affect the properties, productivity and performance of the batteries. Accordingly, it is desirable for the microporous membrane to have resistance to thermal shrinkage, particularly at elevated temperature. Resistance to thermal shrinkage (or “heat shrinkage”) can improve the battery's protection against internal short circuiting that might otherwise occur as the separator shrinks away from the edges of the battery's electrodes at elevated temperature.
European Patent Application Publication No. EP 1 905 586 (published Feb. 2, 2008) discloses multi-layer polymeric membranes useful as battery separator film. One of the membranes exemplified has a transverse direction heat shrinkage of 2% at 105° C.
Japanese patent document JP2000198866 (published Jul. 18, 2000) discloses multi-layer battery separator films having heat shrinkage values of 10%. The membrane comprises layers containing alpha-olefin-CO copolymers and an inorganic species (cross-linked silicone powders).
PCT publication WO2007-049568 (published May 3, 2007) also discloses multi-layer battery separator films having a machine direction heat-shrinkage value of 4% and a transverse direction heat shrinkage value of 3%. The films of this reference comprise a core layer containing heat-resistant polymers or an inorganic filler.
U.S. Patent Publication 2007/0218271 discloses monolayer microporous films having machine and transverse direction heat shrinkage values of 4% or less. The films of this reference are produced from high density polyethylene having a weight-average molecular weight of 2×105 to 4×105, containing not more than 5 wt. % of molecules with a molecular weight of 1×104 or less and not more than 5 wt. % of molecules having a molecular weight of 1×106 or more.
Japanese Patent Application Laid Open No. JP2001-192487 discloses monolayer microporous membranes having transverse direction heat shrinkage values as low as 1.8%, but at a relatively low permeability (Gurley value of 684 seconds). Similarly, Japanese Patent Application Laid Open No. JP2001-172420 discloses monolayer microporous membranes having transverse direction heat shrinkage values as low as 1.1%, but at a Gurley value above 800.
While improvements have been made, there is still a need for battery separator film having increased resistance to heat shrinkage.